(1) Field of the Invention
The present invention relates to a method and apparatus for feedback control of an air-fuel ratio in an internal combustion engine having at least one air-fuel ratio sensor upstream or downstream of or within a catalyst converter disposed within an exhaust gas passage.
(2) Description of the Related Art
Known air-fuel ratio feedback control systems include a single air-fuel ratio sensor system whereby the air-fuel ratio is controlled in accordance with the output of a single air-fuel ratio sensor such as a single O.sub.2 sensor; and a double air-fuel ratio sensor system whereby the air-fuel ratio is controlled in accordance with the outputs of two O.sub.2 sensors upstream and downstream of a catalyst converter (see U.S. Pat. No. 4,739,614). Also, in such a single air-fuel ratio sensor, the above-mentioned O.sub.2 sensor is installed upstream or downstream of the catalyst converter, or within the catalyst converter.
Note that according to a double air-fuel ratio sensor system, the fluctuation of the output of the upstream-side O.sub.2 sensor is compensated by a feedback control using the output of the downstream-side O.sub.2 sensor. Actually, as illustrated in FIG. 1, in the worst case, the deterioration of the output characteristics of the O.sub.2 sensor in a single O.sub.2 sensor system directly effects a deterioration in the emission characteristics. On the other hand, in a double air-fuel ratio sensor system, even when the output characteristics of the upstream-side O.sub.2 sensor are deteriorated, the emission characteristics are not deteriorated. That is, in a double air-fuel sensor system, even if only the output characteristics of the downstream-side O.sub.2 are stable, good emission characteristics are still obtained.
As input circuits for the outputs of the O.sub.2 sensors, use is made of a pull-down type circuit and a pull-up type circuit. The pull-down type input circuit is disadvantageous in that determination of the activation of the O.sub.2 sensor is impossible when the base air-fuel ratio is lean, which will be later explained in detail.
On the other hand, the pull-up input circuit is advantageous in that determination of the activation of the O.sub.2 sensor is possible even when the base air-fuel ratio is lean, but is disadvantageous in that, when the Ohd 2 sensor is short-circuited, determination of the activation of the O.sub.2 sensor is erroneously carried out. Namely, when the O.sub.2 sensor is short-circuited so that the output thereof is grounded (0 V), the output of the pull-up type input circuit is also 0 V, and therefore, the O.sub.2 sensor is determined to be active, and as a result, the air-fuel ratio feedback control by the O.sub.2 sensor is initiated. In this case, since the output of the pull-up type input circuit represents a lean state, the air-fuel ratio may be erroneously controlled toward the rich side, and as a result, the air-fuel ratio feedback control parameter may be adhered to a rich-side guard value, thus increasing HC and CO emissions and reducing the fuel consumption characteristics.